The invention concerns a display based on the principle of electroluminescent polymers with a structured matrix of pixels and a structured second electrode as well as the manufacture thereof.
The graphic illustration of information continues to make strides in our every day lives. Increasingly more objects of daily use are equipped with display elements which enable an immediate retrieval of information required on-site. In addition to the conventional cathode ray tube (“Cathode Ray Tube, CRT”), that delivers a high picture resolution, but which has the disadvantage of heavy weight and high power consumption, the technology of flat panel displays (“Flat Panel Displays, FPD's) was specifically developed for use in mobile electronic devices.
The mobility of the devices places a high demand on the display to be placed in operation. The low weight—which throws the CRT's out of the race from the beginning—should here initially be mentioned. The small structural depth is another essential criterion. Many devices require a display structural depth of even less than one millimeter.
Only low power consumption is required for displays based on the limited capacity of batteries or chargers in mobile devices. Excellent legibility is another criterion, even at great angles between the display surface and the observer, as well as legibility under various ambient light conditions. The capacity to also display multicolored or fully colored information is increasingly gaining in importance. And last but not least, of course, the service life of the components is an important pre-requisite for the use in the various systems. The importance of the individual requirement criteria of the displays always varies in significance in regard to the application scopes.
Several technologies, all of which will not be discussed here, have been established on the market of flat screen displays over a longer period of time. So-called liquid crystal displays (LC displays) are generally dominant today. Despite low-cost manufacturing, low electrical power consumption, light weight and small space requirement, the technology of the LCDs, however, also has serious disadvantages. LC-displays are not self-emitting and are only readable or recognizable under very favorable ambient light conditions. In most cases, this requires background lighting, which multiplies the thickness of the flat screen display. Consequently, the major share of the electrical power consumption is used for lighting and a higher voltage is required for the operation of the lights or the fluorescent tubes. This is generally produced using “voltage-up-converters” from batteries or power packs. Additional disadvantages are the greatly inhibited observation angle of simple LCDs, the extensive switching periods of individual pixels, which is typically at several milliseconds, and [the fact that] they are very temperature sensitive. The delayed screen layout, for example, is extremely adverse in transportation use.
In addition to the LCDs, there are other flat screen technologies, such as vacuum fluorescence displays or inorganic thin film electroluminescent displays. These, however, have not yet reached the required technical degree of maturity or are only conditionally usable for application in mobile electronic devices due to higher operating voltages or manufacturing costs. Displays have made a name for themselves since 1987 based on organic light emitting diodes (organic light emitting diodes, OLEDs). These do not have the above-mentioned disadvantages. The necessity of background lighting is omitted due to the self-emissivity that considerably reduces the required space and the electrical power consumption. The response times are within the range of a microsecond, and they are only slightly temperature sensitive which enables their use in video applications. The reading angle is almost 180°. Polarization foils that are required in LC displays are generally omitted so that a greater luminosity of the display elements may be achieved. An additional advantage is the usability of flexible and non-planar substrates as well as the easy and low-cost manufacture.
Two technologies exist in the OLEDs that vary in type and in the processing of organic material. On one hand, low molecular organic material, such as hydroxyquinolene-aluminum-III-salt (Alq3), that is generally raised to the appropriate substrate by thermal evaporation, may be used. Displays based on this technology are already available commercially and are mostly used in automotive electronics at the present. However, since the manufacture of these components is associated with numerous processing steps under high vacuum, this technology holds presents disadvantages based on high investments and maintenance, as well as a relative low turnover.
An OLED technology has therefore been developed since 1990 that applies polymers as the organic material that may be more chemically wet from a solution to the substrate. The vacuum phases required to produce the organic coatings are omitted in this technique. Typical polymers are polyaniline, PEDOT (manufactured by Bayer), Poly(p-phenylene-vinylene), poly(2-methoxy-5-(2′-ethyl)-hexyloxy-p-phenylene-vinylene) or polyalkylfluorene, as well as numerous derivatives thereof.
The coating structure of the organic light emitting diodes is typically as follows: A transparent substrate (such as glass) is extensively coated with a transparent electrode (such as indium-tin-oxide, ITO). Depending on the application, the transparent electrode is structured with the aid of a photolithographic process that later defines the form of the light emitting pixel.
One or more organic coatings, consisting of electroluminescent polymers, oligomers, low molecular compounds (refer to the above) or mixtures thereof are then applied to the substrate. Polymer substances are generally applied from a liquid phase by spreading or spin coating, as well as by various pressure techniques as of late. Low molecular and oligomer substances are generally separated from the gas phase through evaporation or “physical vapor deposition” (PVD). The overall coating thickness may be between 10 nm and 10 μm and is typically between 50 and 200 nm.
A counter-electrode, the cathode, which is generally of a metal, a metal alloy or a thin insulation coating and a thick metal coating, is applied to these organic coatings. The gas vapor separation, through thermal evaporation, electron beam evaporation or sputtering, is generally used again to produce the cathode coatings.
The challenge in the manufacture of structured displays exists specifically in structuring the above-described coating structure so that a matrix of individual controllable and multi-colored pixels develops. A lithographic technique is available for the first described phase of the OLED manufacture of the structuring of the ITO anode. ITO is extremely insensitive as opposed to the typical spin coatings and developer fluids and may be etched easily by acids such as HBr. Structures with a resolution of a few micrometers may be produced easily by this method.
Structuring the organic coatings and the metal electrode is essentially more difficult. The reason is the sensitivity of the organic material that would be extensively damaged by the subsequent application of aggressive developer fluids or solvents.
The individual functional coatings in OLEDs, based on vaporizable low molecular coatings, may be vaporized on the substrate through a shadow mask so that red, green and blue pixel sections develop. Vaporizing through a shadow mask technique is also available for the striped structuring of the metal cathode (vertical toward the underlying ITO-stripes). This, however, has considerable disadvantages in practical use because of low resolution and the adjustment of the masks over the substrate.
The method of the insulating partitions was therefore developed for this. A series of insulating partitions with a sharp tear-off edge is applied to the substrates directly after structuring the ITO anode. The metal cathode is extensively vaporized after depositing organic coatings (meaning without the use of a shadow mask), whereby the metal film always tears off on the sharp edges of the partitions. This is how separated insulated metal strips (lines) are developed vertically toward the underlying ITO anodes (columns). If a voltage is applied to a particular ITO anode column and a metal cathode line, the organic emitter coating is illuminated at the crossing point between the line and the column. These partitions may have various cross-sections.
Structuring the individual pixels is considerably more difficult for OLEDs based on conjugated polymers that are used up from the liquid phase. Conventional techniques, such as spin coating or spreading, distribute the polymer solution evenly over the entire substrate. A fragmentation into red, green and blue sections with a small structural width, in the event of a color display, is only possible with difficulty, except through subsequent structuring with the assistance of aggressive lithographic methods that considerably damage the polymers.
Several printing techniques have been successfully applied in the past for the structured application of polymers based on this reason. One technique, which has been especially reliable here, is ink jet printing, as well as several versions thereof. However, greater difficulty exists even in these printing techniques to prevent individual adjacent color sections from running into each other. This problem has been circumvented in the past through several solution batches.
European patent 0 892 028 A2 describes a process in which a coating of an insulating material is initially applied onto the ITO-substrate into which windows are inserted in those areas in which the pixels should later be located. This insulating material may, for example, be spin coating that is so modified that it is not moistened by them. The individual drops of the solution (red, green, blue) are also encapsulated at the appropriate points without running into each other and may therefore dry there separately and produce the polymer coating.
This process, however, does not solve the problem of structuring the cathode strips that must be applied onto the polymer as functional coating for passive matrix powered displays. Various technologies have therefore been developed in the past for structuring the cathodes of passive matrix displays. Partitions that are initially applied to the structured ITO substrate were developed in a special process for monochrome displays. The polymer solutions (generally a carrier polymer in a polar solution, followed by an emitter polymer in a polar solution) are spin coated successively onto these substrates. The cathode is then extensively vaporized as a final coating, which tears off from the sharp tear-off edges of the partitions and therefore forms separated cathode strips. This process, however, is initially only suitable for an extensive application and therefore is not suited for full color displays.
An additional coating of the insulating material with “windows” (see above) may be applied as further development of the method of partitions for fully colored displays produced with an ink jet printing process. The insulating windows and partitions are applied to the substrate after applying individual polymer coatings in the process described in European patent 0 951 073 A2. This is again subject to the problems of a treatment of the sensitive conjugated polymers with aggressive developer material, solvents and UV light.
A process is described in patent EP 0 732 868 A2 in which a lithographic treatment of the functional coatings is avoided and a structured cathode is separated at the same time. The partitions for the cathode separation are produced first and then the functional coatings are vaporized in the vacuum through a shadow mask. The serious problem with this method is that the shadow mask does not directly contact the substrate and/or the electrode located on in, but is deposited on the partitions. This considerably reinforces the above mentioned problem of low resolution in the shadow mask technique by vaporizing the mask.
The problem may be summarized in two points. On one hand, a running together of the different colors must be prevented during the structured application. On the other, a structuring of the second electrode must be possible at the same time in passive matrix powered displays.